The invention relates to a steerable drilling method and system for drilling a borehole into an earth formation. The method and system may be used for directional drilling underground boreholes for use as wellbores for the production of hydrocarbons and/or for injecting stimulation fluids into a hydrocarbon fluid containing reservoir formation.
Boreholes are typically drilled using rotary drilling systems with a rotating drill bit and drill string assembly which is rotated by a rotary top drive system at the earth surface. Alternatively or in addition, a downhole drilling motor may be arranged in a Borehole Bottom Assembly (BHA) near the drill bit to rotate the drill bit relative to the drill string.
The drill bit may be steered by positioning a toolface of the the drill bit in a tilted position on the borehole bottom that by both activating the rotary drive and downhole motor the drill bit will make a two superposed rotating motions and drill vertical or straight sections, whereas if the rotary drive rotation is temporary interrupted curved borehole sections may be drilled with a selected angular orientation. In this way a borehole trajectory is drilled with both curved and straight vertical, inclined and/or horizontal sections.
The drill string may be up to 10 kilometers long and comprise 10-15 meters long drill pipe sections that are interconnected by threaded couplings.
The top drive system may provide torque to the drill string to rotate the drill string, which may be twisted so that the top drive has made up to 30 revolutions before the drill bit start to rotate if it is up to 10 kilometers long and may trigger a stick-slip motion of the drill bit, whereby the drill string twist and torque may dynamically and sinusoidally cycle between minimum and maximum values. The top drive system may include a top drive swivel or a rotary table. The drill string transmits the rotational motion to the drill bit. Generally the drill string also transmits drilling fluid to the drill bit to provide cooling to the drill bit, to transport drill cuttings to surface, and for other useful purposes. In order to drill curved wellbore sections that are preceded or followed by straight sections, it has been practised to apply a drill string provided with a downhole motor driving the drill bit, in combination with a rotary drive at surface, whereby the drill bit has an inclined or tilted toolface so that the drill bit is positioned in an inclined or tilted position relative to a central axis of the borehole and borehole bottom.
Rotary Steerable Systems (RSS) are available to the industry for the purpose of steering the drill bit in order to drill a planned well trajectory. Most prior art RSS directional drilling systems use some form of downhole mechanical actuation, such as for example orientation selective force against wellbore formation, by modulating the drilling fluid flow through the mud motor, or by modulating mud flow in bit nozzles. These mechanically actuated directional drilling systems suffer from wear and tear, and often fail under the high temperature, high pressure, high vibrations downhole environment. This leads to expensive pulling of the entire drill string to repair or replace the failed mechanical components at surface.
U.S. Pat. No. 4,485,879 relates to a method for directional drilling of boreholes in subsurface formations by a downhole motor at a lower end of a drill string. The downhole motor rotates a drill bit while a predetermined weight is applied on the drill bit causing the normally-straight axis of the downhole motor to become bent. Simultaneously the drill string is rotated over periods of time that are preceded and followed by selected periods during which the drill string is not rotated. A drawback of this method relates to the friction forces between the drill string and the borehole wall, which are relatively high during periods of time that the drill string is not rotated.
WO-2011130159-A2 discloses a method of controlling a direction of drilling of a drill bit used to form an opening in a subsurface formation includes varying a speed of the drill bit during rotational drilling such that the drill bit is at a first speed during a first portion of the rotational cycle and at a second speed during a second portion of the rotational cycle, wherein the first speed is higher than the second speed, and wherein operating at the second speed in the second portion of the rotational cycle causes the drill bit to change the direction of drilling. This publication also discloses estimating toolface of a bottom hole assembly between downhole updates during drilling in a subsurface formation including encoding a drill string, running the drill string in the formation in a calibration mode to model drill string windup in the formation during drilling operations, measuring a rotational position of the drill string at the surface of the formation, and estimating the toolface of the bottom hole assembly based on the rotational position of the drill string at the surface and the drill string windup model.
U.S. Pat. No. 7,766,098 and U.S. Pat. No. 7,588,100 disclose a system and a method for steering the direction of a borehole advanced by cutting action of a rotary drill bit by periodically varying the rotation speed of the drill bit, either by varying the rotation speed of the motor or by varying the rotation speed of the drill string. It is a drawback of the know system that the speed variation response at the drill bit generally differs considerably from the rotational speed variation at surface due to torsional vibrations and windup of the drill string, particularly for deeper boreholes, resulting in lack of control of the drilling direction.
International patent application WO2011/081673 discloses a method of bit steering by cyclically increasing a rotational speed of a drill bit when an axis of the drill bit is oriented in a desired azimuthal direction relative to a drill string axis.
US patent application US2009/0057018 discloses another directional drilling system wherein an inclined drill bit is steered by periodically varying the rotational speed of the drill string and/or of a downhole drilling motor in the BHA, which is equipped with Measuring While Drilling (MWD) BHA orientation Sensors
US patent application US2009/0065258 discloses a directional drilling method wherein a the rotary speed of a drill string is varied during each revolution and substantially similarly for each of a plurality of revolutions to induce an inclined drill bit at a bottom of the drill string to drill deviated borehole sections with a selected orientation, which is measured using a Measuring While Drilling (MWD) orientation sensing system.
Known MWD systems comprise inclinometers and/or magnetic field detectors to provide a three-dimensional orientation of the BHA and drill bit relative to the earth gravitational and magnetic fields and/or relative to a drill string axis, but do not yet indicate average percentages of time that the BHA rotates through the angular intervals, which is a relevant characteristic for the bit steering process.
There is a need for an improved steerable method and system for drilling a borehole, which overcomes the drawbacks of the prior art and avoids the need for a drill string windup model by monitoring percentages of time that the BHA rotates through the angular intervals, which is a relevant characteristic for the bit steering process.